In the state of the art, active imaging systems, also referred to as laser imaging systems, with a matrix sensor, also referred to as a matrix of detectors or “focal plane array”, are essentially of three types. They all possess a pulsed source of incident light beam illuminating the observed area at each pulse of the incident beam.
A mono-pulse active imaging system (2D flash) with a video rate includes a light source, such as a low-rate solid-state laser, for example of the optical parametric oscillator OPO type. The observed area is illuminated and imaged at each pulse of the incident beam, typically at a rate of 10 Hz to 20 Hz. The images processed in this system are generally submitted to a time aliasing that is a time filtering representative of a deep space filtering.
This first system is limited by the emission mean power of low-rate solid-state lasers, that no proceeds any longer significantly. Such a limitation is all the stronger when the eye safety is required. This requires either the use of an optical parametric oscillator for converting the energy wavelength generally emitted at 1 μm into less dangerous wavelengths, generally ranging between 1.5 μm and 2 μm, or the use of sources directly emitting at the wavelength of 2 μm, less mature than sources emitting at the wavelength of 1 μm.
An active imaging system of the second type includes a pulsed light source showing a better compromise between the emission power on the one hand, and industrial constraints such as the cost, the energy consumption, the bulk and the mass of the source, on the other hand. The source in such a system emits at a higher rate, higher than 100 Hz. The source is for example, according to the applications, a laser diode, a fiber laser or a light emitting diode. This system often operates at a rate ranging between 1 kHz and 100 kHz so as to extract as much light power as possible from the source. The light energy emitted by a pulse being then relatively low, this system digitally cumulates the images associated with a series of pulses to form the final image to be presented to an operator or to an image processing device.
The second type system allows the use of a stronger light source than that used in the first type system. However, this second type system only gains in signal-to-noise ratio or in useful illuminated surface according to the square root of the number of cumulated pulses if the image is read at each pulse of the incident beam before the digital accumulation of associated images. According to a global performance criterion represented by the product of the range at the observed area by the useful illuminated diameter of the observed area, the achieved operation gain is minimum. Moreover, the flow rates of the data to be processed for the reading of the optronic sensor as well as for pre-processing and accumulating images are excessive. Finally, the effective exposure time, corresponding to the total duration needed for forming an image with a sufficient signal-to-noise ratio, is very significantly longer than for the first type system, being a major difficulty in some applications.
The third type relates to an active imaging system having a fast shutter device independent from the matrix of photodetectors, for example a light intensification device. The accumulation of images is analogically achieved by accumulating charges in the photodetectors of the sensor during a series of pulses from the incident beam. The charges accumulated in the sensor are thus only read once per series of pulses through opening and shutting the field of the receiving path upstream the sensor, for example by means of an intensifier.
The analog accumulation system according to the third type gains in signal-to-noise ratio or in useful illuminated surface depending on the mean emission power of the light source. According to the above-defined global performance criterion, the operational gain varies according to the square root of the light emission power. Additionally, the reading rate of the sensor and the flow rate of the data to be processed remain identical to those for a first type system. Nevertheless, such a technique is hard to adapt to all optronic sensors, in particular to avalanche photodetector sensors being particularly promising. In addition, the third type system leads like the second type system to a long effective exposure time, making the system more sensitive to fuzziness resulting from the motion of objects in the observed area or of the system as such, or resulting from strong atmospheric turbulences, than the active mono-pulse imaging, without any possibility of an efficient alignment through processing of images in the case of a third type system. Under impaired atmospheric or submarine conditions generating a high scatter of the incident beam, the second and third type systems also have a limited rate resulting from the to-and-fro propagation time of the light between the system and the observed area.
None of these three types of imaging system can image simultaneously two objects in the observed area should they be separated from an angular distance higher than the divergence of the incident beam. Moreover, the performance of these three types of imaging system is reduced when two objects are to be simultaneously imaged in the observed area if they are separated by a depth distance bigger than the opening time of a time door for integrating the charges of the photodetectors in the sensor, as the space filtering through time aliasing becomes less efficient.
In these three types of imaging system, performance shows to be too limited for meeting some requirements. The available field on one single image remains very low compared to the surface of the areas to be monitored and compared to the fields offered by the visible or thermal passive imaging. The range is not sufficiently larger than that available in passive imaging under clear sky conditions. The second and third type systems are sensitive to fuzziness of the mobility of the system or of objects to be observed.